The absorption of solar Lyman-a (121.6 nm) and X-radiation by the upper atmosphere of the Earth results in the ionized regions known as the ionosphere. The ions and electrons are here present to such an extent that radio-wave propagation is seriously affected and indeed reflection occurs at long wavelengths. The D-REGION lying between about 60 and 90km can be explored from the ground by means of radio propagation techniques, but the effective study of the upper layers (E-, F1-, and F2- regions) require rocket and satellite instrumentation. Above about 500km the density is low plough for the particles to be in orbit about the Earth and here the magnetic field is a dominant influence. This region is the MAGNETO-SPHERE .

In its simplest form, the magnetic field of the Earth may be pictured as a dipole (or straight permanent magnet) displaced about 400km from the centre of the planet. The positive, or north seeking pole, is actually the one on the southern hemisphere and vice versa. The magnetic field lines, which indicate the directions to which a freely suspended magnetic compass would point, emerge vertically at the Earth’s surface at about 75.6°N, 107°W and 66.3o S. 141oK (1965 positions). These positions change slowly with time, as does the intensity of the field. The intensity is presently decreasing at a rate such that, if it continued unchanged, it would be be zero at about AD 4 000; such an extrapolation is not particularly justified however. Studies of partially magnetized rocks have shown that the magnetic field of the Earth has reversed at irregular intervals frequently in the past.

The magnetic field of the Earth strongly influences the charged particles, such as cosmic rays, that are incident on the Earth. Cosmic rays of energies less than a few giga-electron-volts (GeV) cannot approach the atmosphere at mid-latitudes and those that do get down at higher latitudes follow extremely tortuous paths. There are certain regions around the Earth where charged particles can orbit the Earth in relatively stable configurations. These zones have crescent-shaped cross-sections and are regions in which particles become trapped. As they contain a relatively high density of charged particles, they are called the RADIATION BELTS, or VAN ALLEN BELTS after their discoverer .

The particles spiral about the field lines, mirroring back on themselves near the magnetic poles, with periods of 0.1-3 sec for the journey from pole to pole. Superimposed on this rapid motion is a general westward drift of protons, and eastern motion of electrons. Evidence for these radiation belts came from the earliest Russian and American satellites. The work of J. A. Van Alien of the USA and ‘ his colleagues in 1958 provided the first picture of their shape and distribution. Their simple Geiger counters on Explorer 1 registered no charged-particle radiation when they were carried above about 1 000km. Laboratory tests and later satellite experiments showed that the null reading was in fact caused by complete saturation of the Geiger counters. Initially the researchers identified two radiation belts, but such a clear distinction is perhaps misleading. Generally the protons with energies exceeding about 30MeV form a relatively compact belt (the inner belt) centred at about 1.5 Earth radii. The lower-energy electrons and protons occupy a much more extensive zone out to about five Earth radii.

Some of the protons in the inner belt may be indirectly accounted for by higher energy cosmic-ray protons smashing into the Earth’s upper atmosphere. Any neutrons thus released are not influenced by the magnetic field and may stray out to the inner belt region. The half-life of a free neutron is about 12 min; the proton and electron resulting from its decay can be trapped in the radiation belt. The remainder of the particles may be due to the diffusion of cosmic rays and solar wind plasma from interplanetary space.

Several artificial radiation belts were produced in 1958 and 1962 by the detonation of nuclear bombs in space before international agreement to ban such experiments was obtained. The Starfish explosion of 9 July 1962 produced an inner belt that persisted for several years and caused several satellites to become inoperational because their solar cells were damaged. These are probably the most abrupt large-scale environmental changes that Man has made.

If the magnetic field of the Earth faithfully followed that of a dipole even at large distances, then the solar wind would be influenced beyond 100 Earth radii. The pressure exerted by the solar wind is sufficient to compress the Earth’s magnetic field and restrict its influence to about 10 Earth radii to the sunward side of the Earth. In the opposite direction, the solar wind drags the field lines back to distances beyond that of the Moon’s orbit.

Some of the outer field lines in the tail of the magnetosphere (the MAGNETOTAIL) that are closed (that is, run continuously from one hemisphere to another) are so distorted that regions of opposite polarity run parallel to one another and in close proximity. Strong currents can flow in this NEUTRAL SHEET and charged particles can be accelerated when the region is disturbed, as the field lines annihilate and distribute their energy into particle motion. The boundary of the magnetosphere, which as we have seen is far from spherical, is known as the MAGNETOPAUSE . In the same way that a bow wave forms in front of a stick held in a fast-flowing stream, so a shock front stands off the front of the magnetopause. The intermediate zone between the shock wave and the magnetopause is called the MAGNETOSHEATH.

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